Enantioseparation of single layer native cyclodextrin chiral stationary phases: Effect of cyclodextrin orientation and a modeling study

Preparation and evaluation of two chiral stationary phases based on imidazolyl-functionalized bromoethoxy pillar[5]arene-bonded silica

Chiral pillar[5]arene-based mesoporous silica, an emerging class of chiral structure, possesses excellent characteristics such as abundant chiral active sites, encapsulated cavity and excellent chiral modification, which make them a promising candidate as new chiral stationary phases (CSPs) in enantioseparation. In this study, two imidazole-containing (S)-1-(4-phenyl-1H-imidazol-2-yl)ethanamine and (S)-Histidinol were respectively modified to bromoethoxy pillar[5]arene-bonded silica to construct new chiral stationary phases (sPIE-BP5-Sil and sHol-BP5-Sil) for the separation and analysis of enantiomers. The separation conditions such as mobile phase composition, flow rate and temperature were optimized. Under optimal conditions, both sPIE-BP5-Sil and sHol-BP5-Sil showed good separation performance for different types of enantiomers. Interestingly, sPIE-BP5-Sil and sHol-BP5-Sil showed better enantioselectivity for chiral aromatic compounds and chiral aliphatic compounds, respectively. This enantioseparation result was closely related to the presence of additional aromatic rings and abundant hydroxyl groups in the side chains of the two chiral groups. In addition, the enantioseparation process was further studied by molecular docking simulation. Therefore, this work provided a new strategy for the preparation and application of imidazolyl-derived pillar[5]arene-based chiral stationary phases, which can be efficiently used for screening and separating enantiomers.

Investigation of the chiral recognition role of cyclodextrin hydroxyl moieties via high performance liquid chromatography

Cyclodextrin (CD) is known to afford excellent enantioselectivities due to its hydrophobic cavity and external H-bonding sites from hydroxyl moieties. However, there is still a lack of direct and comprehensive evidence clearly illustrating the origin of the important H-bonding effect. Regarding this issue, herein, four allylimidazole CD derivatives by selective substitution of the primary (6-position) and/or secondary (2,3-position) CD were synthesized and clicked onto silica surfaces to afford the corresponding chiral stationary phases (CSPs). The chiral chromatographic performances were systematically evaluated by separating 35 racemic analytes including isoxazolines, dansyl-amino acids, flavonoids and other racemates under reversed-phase HPLC. The chiral selection factors (α) and retention times (k) of the analytes on the as-prepared CSPs were comprehensively compared and it reveals that the enantioseparation ability was significantly altered due to the selective substituents of CD hydroxyl groups. The natural allylimidazole CD CSP (AICDCSP) was superior to the 6-O-tert-butyldimethylsilyl AICDCSP (6-TBDMAICDCSP) for most analytes. Dansyl amino acids and Ar-Pys were well separated on AICDCSP and 6-TBDMAICDCSP, where dansyl amino leucine gained the highest resolution up to 4.72 on AICDCSP, and flavonoids and Ar-Oprs were only separated on AICDCSP. These interesting separation results demonstrate that the secondary hydroxyl groups play a pivotal role in the separation of chiral compounds. In addition, the size of the CD cavity and the choice of solute also have an effect on the separation of substances. The mechanism involved in enantioselective discrimination of the selectively substituted CDs was further investigated by the molecular docking simulation.

Enantioseparation of two antifungal azole drugs by analytical countercurrent chromatography using sulfobutyl ether-β-cyclodextrin as chiral selector

This study reports a successful enantioseparation of two antifungal drugs, Ketoconazole and Voriconazole, using countercurrent chromatography (CCC) with synthesized sulfobutyl ether-β-cyclodextrin (SBE-β-CD) as chiral selector. Two biphasic solvent systems composed of dichloromethane: 0.1 mol L-1 of phosphate buffer solution (pH 3.0) (1:1, v/v) and n-hexane: ethyl acetate: 0.1 mol L-1 phosphate buffer solution (pH 3.0) (1.5:0.5:2, v/v/v) were selected. Influence factors were investigated, including degree of substitution of SBE-β-CD, concentration of SBE-β-CD, equilibrium temperature, and pH of aqueous phase. Under optimized separation conditions, a large enantioseparation factor of α ≥ 3.26 and a high peak resolution Rs= 1.82, was achieved for enantioseparation of Voriconazole by countercurrent chromatography, and purity of two azole stereoisomers collected from CCC separation reached 98.5%, as determined by HPLC. Molecular docking was employed to investigate the formation of inclusion complex.

Vacuum-assisted thermal bonding of β-cyclodextrin and its derivatives as chiral stationary phases for high-performance liquid chromatography

In this work, the vacuum-assisted thermal bonding method was proposed for the covalent coupling of β-cyclodextrin (β-CD) (CD-CSP), hexamethylene diisocyanate cross-linked β-CD (HDI-CSP) and 3, 5-dimethylphenyl isocyanate modified β-CD (DMPI-CSP) onto the isocyanate silane modified silica gel. Under vacuum conditions, the side reaction due to the water residue from the organic solvent, air, reaction vessels and silica gel could be avoided, and the optimal temperature and time of vacuum-assisted thermal bonding method were determined as 160°C and 3 h. These three CSPs were characterized by FT-IR, TGA, elemental analysis and the nitrogen adsorption-desorption isotherms. The surface coverage of CD-CSP and HDI-CSP on silica gel was determined as ∼0.2 μmol m^-2, respectively. The chromatographic performances of these three CSPs were systematically evaluated by separating 7 flavanones, 9 triazoles and 6 chiral alcohols enantiomers under the reversed-phase condition. It was found that the chiral resolution ability of CD-CSP, HDI-CSP and DMPI-CSP was complementary to each other. Among them, CD-CSP could separate all 7 flavanones enantiomers with the resolution of 1.09-2.48. HDI-CSP had a good separation performance for triazoles enantiomers with one chiral center. DMPI-CSP showed excellent separation performance for chiral alcohol enantiomers, among which the resolution of trans-1, 3-diphenyl-2-propen-1-ol reached 12.01. Generally, the vacuum-assisted thermal bonding had been demonstrated as a direct and efficient method for the preparation of chiral stationary phases of β-CD and its derivatives.

Carbon Nanodots from an In Silico Perspective

Carbon nanodots (CNDs) are the latest and most shining rising stars among photoluminescent (PL) nanomaterials. These carbon-based surface-passivated nanostructures compete with other related PL materials, including traditional semiconductor quantum dots and organic dyes, with a long list of benefits and emerging applications. Advantages of CNDs include tunable inherent optical properties and high photostability, rich possibilities for surface functionalization and doping, dispersibility, low toxicity, and viable synthesis (top-down and bottom-up) from organic materials. CNDs can be applied to biomedicine including imaging and sensing, drug-delivery, photodynamic therapy, photocatalysis but also to energy harvesting in solar cells and as LEDs. More applications are reported continuously, making this already a research field of its own. Understanding of the properties of CNDs requires one to go to the levels of electrons, atoms, molecules, and nanostructures at different scales using modern molecular modeling and to correlate it tightly with experiments. This review highlights different in silico techniques and studies, from quantum chemistry to the mesoscale, with particular reference to carbon nanodots, carbonaceous nanoparticles whose structural and photophysical properties are not fully elucidated. The role of experimental investigation is also presented. Hereby, we hope to encourage the reader to investigate CNDs and to apply virtual chemistry to obtain further insights needed to customize these amazing systems for novel prospective applications.

Preparation and chiral resolution properties of bridged bis(cyclodextrin)s hybrid spheres for high performance liquid chromatography

Selenium-bridged bis(β-cyclodextrin)s organic-inorganic hybrid silica material with regular spherical shape as new type of chiral stationary phase was directly synthesized under the one-pot hydrothermal synthesis method, and the chiral stationary phase was further characterized by infrared spectroscopy, scanning electron microscopy, thermogravimetry, and elemental analysis. The results of chiral separation showed that eight chiral compounds including various types of chiral alcohols and flavanone were successfully separated in the reversed-phase separation mode by high performance liquid chromatography, which showed the better chiral resolution effect than that on the C2 position of single β-cyclodextrin. The mechanism of chiral separation was likely due to multiple interactions such as inclusion, hydrogen bonding, electrostatic interaction, dipole-dipole interaction, and the synergistic effect of two cyclodextrins during the chiral resolution process. The synergy of the two cyclodextrins has great potential for development in chiral resolution.

Polysaccharides as separation media for the separation of proteins, peptides and stereoisomers of amino acids

Reliable separation of peptides, amino acids and proteins as accurate as possible with the maximum conformation and biological activity is crucial and essential for drug discovery. Polysaccharide, as one of the most abundant natural biopolymers with optical activity on earth, is easy to be functionalized due to lots of hydroxyl groups on glucose units. Over the last few decades, polysaccharide derivatives are gradually employed as effective separation media. The highly-ordered helical structure contributes to complex, diverse molecular recognition ability, allowing polysaccharide derivatives to selectively interact with different analytes. This article reviews the development, application and prospects of polysaccharides as separation media in the separation of proteins, peptides and amino acids in recent years. The chiral molecules mechanism, advantages, limitations, development status and challenges faced by polysaccharides as separation media in molecular recognition are summarized. Meanwhile, the direction of its continued development and future prospects are also discussed.

Anhydride-linked β-cyclodextrin-bonded silica stationary phases with enhanced chiral separation ability in liquid chromatography

β-Cyclodextrin can be functionalized by derivation of reactive hydroxyl on the ring due to its special chiral environment and structural characteristics, which can be used to identify or separate a variety of chiral substance. In this manuscript, a series of excellent chiral stationary phases for high-performance liquid chromatography were developed for enantioseparation by using anhydride modified β-cyclodextrin bearing chiral (R/S)-α-phenethylamine or (S)-(+)-2-amino-1-propanol. They were characterized by elemental analysis, Fourier transform infrared spectra (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and BET. These chiral stationary phases presented good resolution and repeatability, about 17 kinds of enantiomers were effectively separated. And most of enantiomers were separated better than those reported in the literature in the same both normal and reversed phase modes. The RSD values of R_s for repeatability and column-to-column were below 0.44% and 2.83%, respectively. All results revealed that these new CSPs show great prospect for chiral separation in actual applications.

Click preparation of multiple-thioether bridged cyclodextrin chiral materials for efficient enantioseparation in high-performance liquid chromatography

A highly efficient covalent immobilization procedure is considered as an essential tool for obtaining stable and reliable cyclodextrin (CD) chiral stationary phases (CSPs). This work reports the "thiolene" click immobilization of heptakis(6-mercapto-6-deoxy)-β-CD-CSP onto alkene functional silica to afford novel multiple-thioether bridged CD CSPs by controlling the surface CD concentration. Solid-state NMR, FTIR, TGA and X-ray photoelectron diffraction spectroscopy (XPS) results proved the successful preparation of the desired CSPs with different surface CD loadings. The surface CD concentrations were calculated to be 0.49 and 0.68 μmol m-2 according to the elemental analysis results. More than 60 chiral enantiomers including isoxazolines, chiral lactides, chiral ketones, dansyl amino acids, small molecule acids and alkalis as well as some flavonoids were resolved or partially separated in the reversed-phase HPLC mode. Compared with the previously prepared single thiolene bridged CD-CSP, the current multiple-thioether CD-CSP afforded much better enantioseparation ability due to the existence of the thiol moiety and a confined structure.

Highly efficient preparation of β-CD-based chiral monolithic column by "one-pot" hydroxymethyl polycondensation for enantioseparation in capillary liquid chromatography

A facile strategy for highly efficient fabrication of chiral monolithic column was carried out by the simple "one-pot" hydroxymethyl polycondensation of native β-cyclodextrin (β-CD) and urea-formaldehyde (UF). In-situ rapid substitution of native β-CD and efficient polycondensation of β-CD products and UF oligomers was proposed and fulfilled in one pot. The feasibility, mechanism and key parameters of polycondensation reaction were discussed. The data on crosslinking polycondensation and reaction kinetics were further evaluated. Characteristics including the morphologies, permeability and structure stability of the resultant monolithic columns were also studied. Under the optimal conditions, the "one-pot" hydroxymethyl polycondensation was accomplished in an aqueous solution within only 10 mins. Satisfactory stability and repeatability were gained, and good enantioseparation of eight model enantiomers was successfully achieved with the resultant β-CD-based monolith. It was simple and highly efficient, and the organic solvents, special CD derivatives and tediously long-time polymerization reaction were successfully avoided, which might light a new access to rapid preparation of CD-chiral monolith for high-performance enantioseparation.

Chirality Discrimination at the Single Molecule Level by Using a Cationic Supermolecule Quasi-Gated Organic Field Effect Transistor

Achieving rapid and highly sensitive small molecule chiral discrimination is a great challenge in modern-day analytical sciences. Herein, an organic field effect transistors (OFET) is developed by employing an imidazolium 3,5-dimethylphenylcabamoylated-β-cyclodextrin (Im⁺-Ph-β-CD) as both the recognition unit and a quasi gate, which induces a secondary accumulation channel of electrons in the n-type transistor to achieve the signal transduction and amplification via field effect. The charge of the imidazolium group is partially shielded due to its self-inclusion in the CD cavity, and this shielding effect is reduced at varying degrees in the existence of isomers due to the competitive inclusion. Consequently, the different weak intermolecular interactions related to the target-induced CD-enantiomer complexation with different geometry and stabilization energy for each isomer can be transformed to electronic signals based on the variety of Im⁺-Ph-β-CD's effective charge rather than the intrinsic charge of analytes, hence leading to chiral differentiation, and the hydrogen-bonding network of Im⁺-Ph-β-CD membrane further magnifies the signal. This working strategy even allows chiral discrimination of electrically neutral analytes. The as-prepared sensor affords rapid and real-time discrimination to small molecule enantiomers at single molecule level with a limit of detection of 8.1 × 10^-19 M in a 200 μL volume (about 100 small molecules). Moreover, we prove the great potential of the chiral organic field effect transistor in quantitative analysis of commercial medicines.

Perylene-functionalized graphene sheets modified with β-cyclodextrin for the voltammetric discrimination of phenylalanine enantiomers

A facile approach was reported to synthesize β-cyclodextrin functionalized graphene that is bridged by 3,4,9,10-perylene tetracarboxylic acid (rGO-PTCA-CD) via a chemical route that involves the functionalization of rGO with PTCA followed by covalently cross-linking NH₂-β-CD. The as-prepared rGO-PTCA-CD was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical methods. The working electrodes were thoroughly studied for the cyclic voltammetry by using [Fe(CN)₆]^4-/3- as redox probe and using ferrocene as an internal standard. Furthermore, rGO-PTCA-CD was successfully applied to the recognition of phenylalanine enantiomers. The host-guest inclusion interaction between rGO-PTCA-CD and the phenylalanine enantiomers was investigated by differential pulse voltammetry with Fc used as a competitor. The recognition result showed that the rGO-PTCA-CD-modified glassy carbon electrode exhibited higher chiral recognition capability for L-Phe than for D-Phe with an enantioselectivity coefficient of 2.07. The proposed modified electrode had a limit of detection of 0.08 nM and 0.2 nM (S/N = 3) for L-Phe and D-Phe, respectively, with a linear response range of 0.01 mM to 5 mM, which was ascribed to the synergy of the rGO-PTCA (e.g., its excellent electrochemical performance) and β-CD (e.g., the hydrophobic inner cavity with good molecular recognition and enrichment abilities).

Graphene quantum dots functionalized β-cyclodextrin and cellulose chiral stationary phases with enhanced enantioseparation performance

Graphene quantum dots (GQD) functionalized β-cyclodextrin (β-CD) and cellulose silica composites were first prepared and applied in HPLC as chiral stationary phases (CSP) to investigate the effect of GQDs on chiral separation. Through comparing the enantioseparation performance of GQDs functionalized β-CD or cellulose CSPs and unmodified β-CD or cellulose CSPs, we found GQDs enhanced the enantioseparation performance of nature β-CD, β-CD-3,5-dimethylphenylcarbamate derivative and cellulose-3,5-dimethylphenylcarbamate derivative. Molecular modeling was applied to understand and theoretically study the enhancement mechanism of GQDs for enantioseparation. According to molecular simulation results, GQDs provide extra interactions such as hydrophobic, hydrogen bond and π-π interaction when chiral selector interacts with enantiomers, which enhances the chiral recognition ability indirectly. The molecular simulation results showed a good agreement with the experimental results. Our work reveals the enhancement performance of GQDs for chiral separation, it can be expected that GQDs-based chiral composites and chiral GQDs have great prospect in chiral separation and other research fields such as asymmetric synthesis, chiral catalysis, chiral recognition and drug delivery.

Wide pH range enantioseparation of cyclodextrin silica-based hybrid spheres for high performance liquid chromatography

pH plays an important role in the enantiomeric separation process by changing the polarity of the mobile phase and the conformation and ionization state of the enantiomers. Herein, β-cyclodextrin-silica hybrid spheres with n-propyl groups as hydrophobic linkers and ethyl-silica as a support were prepared using a one-pot approach, and then the hydroxyl group was further modified with 3,5-dimethylphenyl isocyanate. The new β-cyclodextrin-silica hybrid chiral stationary phase (CD-HCSP) was prepared and characterized using techniques including scanning electron microscopy and transmission electron microscopy. The enantioseparation properties of CD-HCSP were evaluated with different solvents over a wide pH range (1-10) in reversed phase. 14 enantiomers were successfully resolved, and favorable chiral resolution and high stability was demonstrated for multiple types of enantiomer under different pH conditions. Compared with commercial columns, CD-HCSP showed better chiral resolution and, more importantly, could be used for chiral resolution over a wide pH range. This work combines the high pH tolerance of the hybrid material and excellent chiral recognition of cyclodextrin for enantioseparation of chiral drugs, which could lead to the development of a new type of chiral separation material.

Recent studies of docking and molecular dynamics simulation for liquid-phase enantioseparations

Liquid-phase enantioseparations have been fruitfully applied in several fields of science. Various applications along with technical and theoretical advancements contributed to increase significantly the knowledge in this area. Nowadays, chromatographic techniques, in particular HPLC on chiral stationary phase, are considered as mature technologies. In the last thirty years, CE has been also recognized as one of the most versatile technique for analytical scale separation of enantiomers. Despite the huge number of papers published in these fields, understanding mechanistic details of the stereoselective interaction between selector and selectand is still an open issue, in particular for high-molecular weight chiral selectors like polysaccharide derivatives. With the ever growing improvement of computer facilities, hardware and software, computational techniques have become a basic tool in enantioseparation science. In this field, molecular docking and dynamics simulations proved to be extremely adaptable to model and visualize at molecular level the spatial proximity of interacting molecules in order to predict retention, selectivity, enantiomer elution order, and profile noncovalent interaction patterns underlying the recognition process. On this basis, topics and trends in using docking and molecular dynamics as theoretical complement of experimental LC and CE chiral separations are described herein. The basic concepts of these computational strategies and seminal studies performed over time are presented, with a specific focus on literature published between 2015 and November 2018. A systematic compilation of all published literature has not been attempted.

Recognition Mechanisms of Chiral Selectors: An Overview

Stereospecific recognition of chiral molecules plays an important role in nature as the basis of the interaction of chiral bioactive compounds with the chiral target structures. In separation sciences such as chromatographic and capillary electromigration techniques, interactions between chiral analytes and chiral selectors, i.e., the formation of transient diastereomeric complexes in thermodynamic equilibria, are the basis for chiral separations. Due to the large structural variety of chiral selectors, different structural features contribute to the overall chiral recognition process. This introductory chapter briefly summarizes the present understanding of the structural enantioselective recognition processes for various types of chiral selectors.